1. Field of the Invention
This invention relates primarily to equipment used for effecting temporary direct contact of two liquids which are substantially mutually insoluble.
2. Description of the Prior Art
Mutually insoluble liquids are usually brought into direct contact with each other for any of three principal purposes, namely, transfer of dissolved material or heat from one liquid to the other, or to cause a chemical reaction between the two liquids. The most important of these purposes, and the one with which this invention is primarily concerned, is solvent extraction, i.e., the separation of solution components by causing their unequal distribution between two mutually insoluble liquids. As used herein, the phrase "mutually insoluble liquids" means two liquids of such limited solubility in each other that for practical purposes they are considered mutually insoluble. Water and oil are examples of two such liquids.
Solvent extraction processes, particularly in the mining industry, typically use multiple stages of mixer-settler units, with an aqueous phase and a water-insoluble organic phase, such as, kerosene, being repeatedly mixed and separated, the two phases flowing countercurrently between stages of mixing tanks and settling tanks. One common version provides mixing and the pressure differential required for flow with a simple centrifugal pump impeller which mixes the aqueous and organic phases together as they enter a mixing tank, and pumps the mixture into a settling tank where the two phases separate and are separately recovered.
Considerable work in both the chemical and mining industries has led to the generally accepted view that maximum uniformity of mixing is achieved when the depth and diameter of the mixing tank are approximately equal. Accordingly, it has been the practice of the prior art to use tanks of roughly equal depth and diameter for mixing. A typical mixing tank used in extracting copper from a copper-bearing ore is 12 feet in diameter and 12 feet high. Aqueous and organic phases enter at the bottom of such a mixing tank, and are mixed and pumped into a settling tank, which has to be at or near the elevation of discharge from the mixing tank for maximum pumping efficiency and to avoid excessive pumping action which would produce an unacceptable over-emulsification of the aqueous and organic phases. A typical settling tank is in the shape of a flat horizontal pan about 50 feet long, 50 feet wide, and about 2 feet deep. Therefore, the bottom of the settling tank is about 10 feet above the bottom of the mixing tank. The liquid in the settling tank imposes a load of about 100 lbs. per sq. ft., resulting in a total load of about 250,000 lbs. on a typical settling tank, which must be supported about 10 feet in the air.
Thus, conventional mixing tank proportions applied to the design of mixer-settler units for solvent extraction require that the settling tanks be substantially above ground level, and the required elevation increases with increasing flow rates through the mixer-settler units. Such construction is expensive because of the cost of erecting elevated supports and providing necessary stiffening for the settling tank bottom.
Another disadvantage of the settling tank supported in the air is that it is subject to damage due to earthquakes and uneven ground settling, which can produce destructive tipping moments.